The present invention relates to an air-fuel ratio control apparatus for detecting the air-fuel ratio of the mixture from the composition, e.g., the concentration of oxygen in the exhaust gas from an engine and feedback controlling the air-fuel ratio so as to maintain it at the desired ratio.
In a known apparatus of this type including an oxygen concentration detector for detecting the air-fuel ratio, a comparison circuit responsive to the signal from the oxygen concentration detector to make a comparison and determination whether the air-fuel ratio is greater or smaller than a predetermined air-fuel ratio, an integrating circuit responsive to the comparison output to generate an integration voltage and air-fuel ratio control means, e.g., a driver circuit for controlling the amount of air bleed to the carburetor in accordance with the integration voltage as an air-fuel ratio correction amount, the feedback is applied in accordance with a predetermined control constant irrespective of whether a low region or a high region. However, this method is disadvantageous in that the air-fuel ratio becomes richer in the high region than in the low region so that the level of the controlled bleed amount is increased and the bleed sensitivity (a change of the air-fuel ratio in response to a change in the flow rate) is deteriorated thereby deteriorating the control response characteristic.